A combine harvester incorporating a grain cleaning system as summarised above is known from U.S. Pat. No. 4,344,443 which for convenience is incorporated herein by reference to avoid the need to describe a complete combine harvester. The present invention provides an improvement over the grain cleaning system shown in FIG. 2 of the latter patent.
As described in greater detail in U.S. Pat. No. 4,344,443, in a combine harvester, crop is cut and advanced by a header into an elevator assembly which feeds the crop into a threshing mechanism. The latter beats the crop to separate the bulk of the grain from the straw, the grain dropping onto a grain pan and the straw being transported by a straw walker to the rear of the combine harvester. As the straw is moved along by the straw walker, some more grain may drop from it and be guided by a sloping tray onto the grain pan. The straw can be dropped in a swath behind the harvester or it may be spread evenly over the ground, sometimes after it has been chopped.
Aside from the grain, chaff drops onto the grain pan and needs to be separated from it. The purpose of the grain cleaning system is to separate the grain from the chaff so that only the grain may be collected and conveyed by an auger to the grain storage tank of the combine harvester while the chaff is discarded.
The grain pan is a non-perforated tray made of sheet metal with transversely extending corrugations. The pan is reciprocated to cause the grain to hop towards the rear of the harvester, eventually to fall onto a reciprocating sieve, usually the upper of a stack of two or more sieves. Conventionally, the sieves comprise series of pivotable louvers, which may be set to a range of sieve openings. During their transport along the grain pan, the grains tend to migrate to the bottom of the grain pan, such that the upper portion of the crop layer at the end of the grain pan primarily contains lighter chaff particles.
A blower blows air onto the grain and the chaff as they fall from the grain pan onto the upper sieve and also blows air through the sieves. This keeps the less dense chaff in suspension while allowing the individual grains to drop between the sieve louvers into a collector. The reciprocation of the sieves once again causes the material to hop and to migrate towards the rear of the combine harvester. The tossing of the grain by the sieves tends to separate individual grains from any husks surrounding them but some grains may still remain trapped in incompletely threshed ears, termed the tailings, which fall off the rear end of the upper sieve. Such tailings are separately collected at the bottom of the cleaning mechanism and transported by a second auger for re-processing.
The overall capacity of combine harvesters is constantly being increased to meet customer demand and this places ever greater requirements on the grain cleaning system. To increase cleaning capacity larger accelerations need to be applied to the sieves, which themselves need to be constructed more sturdily to withstand the additional weight of the grain. However, vigorous vibration of large out of balance masses causes operator discomfort and can lead to breakdown through fatigue in the mechanical components of the cleaning system.
For efficient grain cleaning it is primordial to assure a steady transport and a continuous layer of chaff and grain mixture along the sieves. Otherwise heaps of crop material will develop which can no longer be penetrated by the air flow to discard the chaff particles, while the air tends to escape through the lightly loaded sieve sections. Such situations are harder to avoid when the overall crop load increases.